1. Field of the Invention
The present invention relates to cards containing infrared-absorbing bar codes and more specifically to bar-coded cards wherein the bar code can be perfectly concealed so as not to be visible.
2. Discussion of the Related Art
Recently, as a measure against tampering of magnetic records on private cards, ID cards and the like, it is practiced to print fixed information, such as magnetic bar codes or infrared-absoring bar codes, and provide thereon a covering layer or other covertures to conceal these bar codes. For example, the whole area of the infrared-absorbing bar code printed on a voting card is covered up by smearing with an ink which has high light absorptivity in the visible region but low light absorptivity in the infrared region (Japanese Patent Application Kokai No. 58-45999). This method, however, had the problem of a disagreeable visual image because of the black color of the ink which has low light absorptivity in the infrared region. This method also had the problem that it was incapable of perfectly concealing the bar code in order that it would be invisible to the naked eye.
In order to eliminate these problems to improve image and perfectly conceal the bar code so that it is invisible to the naked eye, it has been tried to cover up the infrared-absorbing bar code by smearing with an ink having high light absorptivity in the visible region but low light absorptivity in the infrared region and printing thereon with a white ink comprising titanium oxide to form a white concealing layer. It has further been attempted to print a pattern on the white concealing layer.
However, even if such a white concealing layer is provided by printing with a white ink comprising titanium oxide, it is impossible to attain perfect concealment of the infrared-absorbing bar code in order that it will be invisible to the naked eye. Also, when the white concealing layer is made thick or additional printing of a pattern is made thereon to assure perfect concealment of the infrared absorbing bar code, there arises the problem that when the bar code-printed objects are placed one upon another or piled up, the bar code-printed portions become bulky or bulgy, giving rise to trouble in handling or use of the objects (such as cards).